The venerable BNC connector is used in a great many instruments for a variety of purposes, ranging from connecting probes on equipment that uses probes (e.g., oscilloscopes) to general input and output of signals. It comes in various grades, ranging from the more expensive instrument grade clamp type to crimp on versions that are not expected to exhibit the full measure of performance or long term durability that is associated with the silver plated mil-spec top of the line versions. Aside from the variety of versions to choose from, one of the positive aspects of BNC is its ease of use. It is pushed on and then mated with a simple quarter-turn twist. This aspect of BNC compares favorably with other series connectors, such as TNC, N, SMA, APC-7 and APC-3.5, each of which involves a threaded nut or sleeve that must be given several turns to mate the connector.
BNC connectors are readily available, relatively inexpensive (as RF connectors go) and all the various versions (as long as the characteristic impedance is the same) inter-mate with one another. It is truly a workhorse of the electronics industry.
Despite its popularity, the BNC connector has some significant drawbacks when used as an instrument grade connector for some electronic test equipment, such as top of the line high frequency oscilloscopes. It has reactive discontinuities at high frequencies. That is, above certain frequencies it fails to match the 50 xcexa9 characteristic impedance of the coaxial transmission line of which it is expected to be a part. Even the most carefully installed mil-spec clamp type BNC connector is extremely visible as a discontinuity on a TDR (Time Domain Reflectometer) of even modest bandwidth. Next, it tends to xe2x80x9cleakxe2x80x9d (radiate from its mating surfaces) above, say, 500 MHZ. Finally, since it relies solely on internally supplied spring tension to draw its parts together, it can, when under externally applied tension, allow the mating parts to separate sufficiently to degrade the quality of the connection (greater discontinuity, more loss), sometimes to point where the connection is interrupted altogether (especially if the parts are worn from extended use).
Much (although not all) of the problems of BNC connectors can be traced to aspects in the design of the male half, which is to say, the part that has the male center conductor pin and that is given the quarter turn twist while gripping a knurled shell we shall call a bayonet latch. Let us briefly take a closer look at the conventional BNC connector, the better to appreciate why it has these problems.
To begin, refer to FIG. 1, which is a side view 1 of a pair of mating BNC connector halves 2 and 3. Connector half 2 is the female part (going by the pin for the center conductor, which is not visible), while connector half 3 is the male part. From the drawing we cannot tell what overall function the female part 2 performs; it might be part of a xe2x80x9ctee,xe2x80x9d the BNC part of a cross series adapter, be cable mounted or bulkhead mounted; such differences do not matter, the portion that is shown would be the same in all those cases. The male part is depicted as being a clamp-type cable mount part, and that, too, is simply one choice among many. Thus, a rear portion of a male shell 10 of the clamp variety is visible, as is the cable 11 it attaches to.
The female part 2 has a reduced diameter section 5 over which the male part slides. During this action the male center pin enters the female center pin (which is not visible in this view) while spiral grooves 7 engage pins 6 for the quarter turn (another groove 7 and pin 6 are on the back side of the part shown). A detent 8 in the spiral groove 7 engages the pin 6, preventing spontaneous disengagement by requiring that some twisting force be applied to the connector to overcome the action of the detent. A spring action to be described below cooperates with the detent and pin 6 in producing this behavior. A knurled ring on bayonet latch 9 assists in performing the quarter turns needed for mating and un-mating the connector halves.
Refer now to FIG. 2, which is an exploded view of the BNC connector halves of FIG. 2. While bearing in mind that BNC connectors are fabricated in different ways according to manufacturer""s preference, we can nevertheless appreciate the basic mechanisms that account for certain of the BNC connector""s troubles that we set out above.
Note that the female connector portion 2 includes a female center pin 13 that is centered and held in place by a Teflon sleeve 15. The sleeve 15 has a stepped diameter in front that engages a corresponding shoulder in a female shell (4, 5). The sleeve 15 is secured in place from the rear, either by a rolled edge with or without a washer, or, as is shown, by a clamp nut 14. (In this figure we have chosen to let connector half 2 be a bulkhead mounted clamped-to-cable assembly, which is merely exemplary.) Teflon sleeve 15 has a reduced diameter portion 22 that will be of interest, shortly.
Now consider the male connector half 3. As an assembly it includes a Teflon sleeve 20 whose rear portion has a small diameter bore that centers and supports a male center pin 19, and whose front portion has a larger diameter bore 23 sized to just slip over the reduced diameter portion 22 of sleeve 15.
Another aspect of male connector half 3 that is of interest are the washers 16 and 18, between which are located spring washer(s) 17. When assembled, the knurled male BNC bayonet latch 9 is made captive to male BNC shell 21. BNC shell 21 has a collection of slits that make somewhat springy the end that enters the female shell 5.
Here is how BNC shell 21 is made captive to the bayonet latch 9. Washer 18 centers itself on and is retained by, a shoulder 24 of the male shell 21 (or the washer 18 is split, so that it may snap into a groove 24), while the outside of washer 16 centers itself within a cavity in the back of the bayonet latch 9, as its inside slides over the outer diameter of male shell 21. A rolling of an edge in the back side of bayonet latch 9 makes washer 16 captive within the cavity. Furthermore, bayonet latch 9 cannot slide off the male shell toward the rear, owing to a stepped shoulder 24. Between the two washers 16 and 18 are one or more spring washers 17 that push washers 16 and 18 apart. What they also do is push the bayonet latch 9 rearward in the direction of arrow 25 when the connector halves are mated. This is the internally supplied spring tension that keeps detent 8 engaged against pin 6, and requires torque to overcome in order to unlatch the connector. Unfortunately, pulling on the cable 11 (see FIG. 1) in the direction of arrow 25, or otherwise inducing external tension urging the two connector halves apart, can overcome the internal spring tension (from spring washers 17) keeping the connectors halves together. A sufficient tension will compresses the springy washers 17 as they yield, and the connector halves draw apart slightly.
There are two basic aspects that we wish to point out. First, the tapered end of the male center pin 19 enters a slit end of female pin 13, and ordinarily spreads those slit portions apart slightly, for good contact. As the connector wears the resilience in the slit female pin is reduced, so that a slight withdrawal of the male pin can decrease the ohmic quality of the connection. Equally as bad at higher frequencies, as the withdrawal occurs, there appears a short length over which there is a decrease in diameter. (That is, the male and female center pins have the same outer diameter, and when they are fully mated there are annular surfaces that touch, shoulder to shoulder. When that occurs there is no, or very little, effective change in the outer diameter of the combined center pins.) A similar increase in the effective diameter of the outer conductor occurs also, as the end of the male shell pulls away from the shoulder in the female shell that it seats upon. These changes are important, since the characteristic impedance of a coaxial transmission line involves the ratio of the outer diameter of the center conductor and the inner diameter of the outer conductor, as moderated by the dielectric constant therebetween. When the male center pin 19 withdraws slightly from the female pin 13, a short length of diameter reduction occurs at the same location as a short length of outer diameter increase occurs, and this xe2x80x9cdouble whammyxe2x80x9d appears as a very definite discontinuity.
A similar bad thing happens in connection with the Teflon sleeves 15 and 20. Ordinarily, the reduced diameter section 22 of sleeve 15 would be the exact complement of the large diameter portion 23 of sleeve 20. The idea is that when they mate their edges vanish, as it were, and the two parts act as a single part of continuously present material of the proper diameter. (The joining of the center conductor pins is supposed to use this same idea.) That fails when the connector halves pull apart, producing a discontinuity owing to a location of altered dielectric constant. Furthermore, the presence of the Teflon is a bit of a problem in the first place, since it is difficult to machine the stuff to the tolerances needed to reliably perform the magic of the vanishing edges. Also, it is the Teflon that is supposed to hold the center conductor pins in their proper locations, but it can""t hold tight tolerances. Especially since Teflon cold flows so easily; even a brand new, but especially a used connector, will have Teflon sleeves 15 and 20 that exhibit and account for significant mating anomalies.
It would be desirable if there were a precision BNC connector whose male and female portions would inter-mate with their conventional counter-parts and perform satisfactorily, but that when mated with themselves gave truly superior performance, right up to 18 GHz, comparable to the best precision type N connectors. Ah, but how to do it?
A solution to the problem of poor RF performance in the conventional BNC connector is to first, eliminate the use of Teflon, in favor of an air dielectric in the vicinity of the mating parts, and support the male and female center pins further back within the body of the connector, using other proven dielectric materials borrowed from the precision type N connector, or from another 7 mm RF connector. Next, a captive knurled draw nut provides positive displacement and the tension needed to draw the already mated male and female connector halves together, in place of the conventional spring tension. It is the bottoming out of the male shell inside the female shell that resists the positive displacement and the tension supplied by the knurled draw nut, ensuring that the two connector halves are actually in contact, and that the edges of surfaces that need to xe2x80x9cvanishxe2x80x9d for good operation do indeed vanish. The mating center conductors are rigidly mounted within their shells and bottom out against each other at the same time as do the shells. The basic bayonet latch mechanism is retained, so that either half of the new connector will mate with opposite sex halves of conventional BNC connectors.